A visual analog scale (VAS) is a psychometric instrument which can be used to measure any scalar quantity over a range, but it is particularly useful for self-rating of feelings, attitudes and intensity of subjective experiences. Changes in these subjective states are difficult to measure by an objective means. Psychologists call the underlying subjective experiences such as pain, anger, sadness, “latent variables,” and rely on self-assessments to quantify degree, recognizing that the self-assessment is a report about a variable that is not otherwise directly corroborated. For example, the VAS is commonly used in patient health assessments to measure pain intensity or pain relief felt by a patient after a medical procedure, or due to an illness, as part of a treatment or clinical trial. A VAS comprises a vertical or horizontal line, where the endpoints of the line define the extreme values of a psychological range for the scale. For example, if the VAS is being used to measure pain intensity, then one endpoint of the VAS might represent no pain (e.g., scored as 0) and the other endpoint might represent extreme pain (e.g., scored as 10 or 100).
Traditionally, a VAS has appeared, as a vertical or horizontal line on a paper form, as shown in FIG. 1. The VAS 100 includes left and right anchor lines at the scale extremes, 120 and 130 respectively, at either end of the line, with text (“anchor text”) describing the level of feeling, sensation or subjective state corresponding to each extreme. The VAS 100 is divided into intervals of distance. Each interval represents a unit value, in proportion to the distance between the two extreme values at the anchors (e.g., between 0 and 10 or 0 and 100). It is important that the length of each interval of distance along the VAS be identical in length to ensure that each unit value has an equal probability of being selected by a subject. Typically, a “stem” (i.e., a question or request intended to elicit a response from a human subject) on a VAS appears above or below the VAS. Subjects indicate a response to the stem by placing a pen or pencil mark that crosses the line somewhere along the VAS. Their response quantifies their subjective felt state in relation to the two extreme psychological states that are described at the ends of the VAS line. For example, a stem question may ask how much pain a subject has felt in the past 24 hours. If the subject felt mild pain, he can put a mark close to the “no pain” endpoint to represent his assessment of the experienced pain.
In designing a VAS, there are certain variables (e.g., VAS length, total number of units, and labeling language) that need to be defined in order to relate the VAS response to a score. Consistency in these variables allows VAS results to be compared across subjects in a single or multiple studies, or across the same subject over a period of time. The paper version VAS is typically 10 centimeters (cm) long and represents a score ranging from 0 to 100 (or 0.0 to 10.0), with 100 intervals of distance. Because survey instruments eliciting VAS responses are administered to subjects on fairly standard paper size, replicating a 10 cm long VAS including 101 values corresponding to 100 identical intervals of distance is practical. A subject's response on the VAS can be scored numerically by applying a commonly available metric ruler to measure the distance from one end of the scale to the subject's response mark. Because the intervals of distance on the VAS are the same size and each interval of distance corresponds to a single score, each score is equally likely to be chosen. In contrast, if each interval of distance corresponding to a score was not equally sized, then scoring bias would result because some scores would correspond to a greater interval of distance. Thus, equally sized intervals along the VAS line support consistent and unbiased scoring results.
However, the implementation of VAS assessments is shifting away from paper form to electronic devices such as hand-held computing devices or tablets. While displaying a VAS on a hand-held or other computing device supports automatic scoring and may be easier for subjects and scientists to use, these new methods of implementing and administering VAS scales also introduce challenges. In contrast to the standard sized paper, computing devices come in different sizes, some with display areas smaller than 10 cm. So a standard 10 cm long VAS simply will not fit on such a small display. Further, the diversity of device display sizes (from 2-20 cm or even more) can support VAS lines of varying length suited to each display. However, in displaying a VAS on a computing device, one objective remains the same as with its paper counterpart: to avoid bias in the scoring of subject responses.
One approach to dealing with this problem is to display a VAS that has a fixed, pre-programmed, length regardless of the aspect ratios, resolutions and sizes of different screens. In order to divide this fixed length into 100 intervals, a computing device might adjust the number of pixels assigned to each of the 100 intervals along the VAS line so that the intervals are unequal to each other. However, the result would then be that some intervals on the VAS line would be longer than others and thus more likely to be selected, introducing a bias in favor of the intervals with more pixels. Thus, there is a general need for an improved method for displaying a VAS on a computing device. Embodiments of the disclosed subject matter are directed to a VAS displayed on a computing device that automatically adjusts to different screen aspect ratios/resolutions so as to avoid bias in the scoring of a response line.